Storm Forecast

Storm Forecast
Valid: Tue 25 Jun 2013 06:00 to Wed 26 Jun 2013 06:00 UTC
Issued: Mon 24 Jun 2013 21:34
Forecaster: PISTOTNIK

A level 1 was issued for parts of Bulgaria, Romania, Moldova, the Ukraine and Southeastern Poland for excessive precipitation and to a lesser extent for large hail.

A level 1 was issued for Northeastern Poland, the Russian exclave of Kaliningrad and parts of Lithuania for excessive precipitation and to a lesser extent for tornadoes.

A level 1 was issued for Northern Sweden for large hail and severe wind gusts.

SYNOPSIS

A long-wave trough is situated over Central Europe and a closed upper-level low over the Czech Republic, which brings these regions a return of below-average temperatures and plentiful (stratiform) precipitation after last week's heat wave. Postfrontal cool and rather dry air also moves southward into Western Europe and even the Mediterranean region, while it becomes exposed to subsidence in the neighborhood of subtropical high pressure.
This pattern effectively pushes the Saharan elevated mixed layer back to where it came from. Steeper lapse rates will only keep their last bastions over Southern Iberia, Greece and Turkey, but a strong cap and a lack of forcing will seasonably preclude any convection over most of the Mediterranean Sea despite some limited CAPE signals.
Further downstream, warm to hot air continues to move northward over Eastern Europe, Russia and even Scandinavia, bounded to the West by a wavy and slowly advancing cold front. The main focus for deep convection is provided by a low-level cyclogenesis over Poland, which simmers at medium level ahead of a vorticity lobe that travels northward from Serbia and Western Romania.

DISCUSSION

... Russia, Eastern Europe ...

In the absence of steep lapse rates, it is left to the amount of low-level moisture to discriminate between regions bereft of CAPE (Belarus, Eastern Ukraine) and regions with substantial CAPE (basically, the rest) within the homogeneously warm air mass. Instability is maximized in the prefrontal belt from Romania to the Baltics, which was not sampled by Monday's soundings. However, observed dew points of 18 to 20C were in line with the model predictions and make the forecast CAPE in the 1000 to 2000 J/kg range on Tuesday look realistic again. Most of the unstable regions are placed under very weak vertical wind shear, which only increases to values around 20 m/s near the Western fringe where a moderate Southerly jet streak emerges from the nose of the long-wave trough.
Scattered to widespread thunderstorms will form again on Tuesday over many portions of Western Russia and Eastern Europe. Similar to Monday, the highest coverage is expected in the corridor spanned by the maximum of prefrontal CAPE and the cold front, where a level 1 area was drawn from Bulgaria into Poland. Precipitable water between 30 and 40 mm, storm motion vectors almost parallel to the cold front and chances for repeated rounds of thunderstorms point to a distinct risk of excessive precipitation. Isolated large hail is possible as well with stronger pulse storms, fuelled by strong CAPE release towards the East and by increasing (unidirectional) vertical wind shear towards the West. However, poor mid-level lapse rates and a rapid clustering of storms are thought to be somewhat counterproductive to a higher hail risk.
Further North, the warm front of the Polish low will move from Northeastern Poland, Kaliningrad and Lithuania out to the Baltic Sea until noon. In the warm sector, increasing Easterly surface winds are forecast to advect the richest low-level moisture back to the coast and to improve low-level veering (0-1 km shear >10 m/s and 0-1 km SRH >100 m^2/s^2). A slightly enhanced tornado risk cannot be discounted, even though the overall magnitude of deep-layer shear is lower than further South and the best helicity may be associated with still too cool boundary layer to enable surface-based storms. All in all, though, the rain risk was the main reason to extend the level 1 area further to the North.

... Scandinavia ...

Warm and moderately moist air will likely allow limited CAPE build-up, and scattered thunderstorms are forecast to form in response to diurnal heating. As in Eastern Europe, vertical wind shear is weak across most of the region and only increases in closer proximity to the long-wave trough towards the West.
Main feature of interest is a pronounced short wave, which is embedded in the increasing Southwesterly flow and crosses Northern Norway and Sweden in the afternoon hours. Since observed dew points on Monday (12 to 16C) were higher in that area than forecasted by GFS, confidence increases in an overlap of at least low-end CAPE (200-500 J/kg), fairly strong deep-layer shear (20-25 m/s) and ample forcing. Scattered storms can organize into multicells or even an isolated supercell. A level 1 was drawn for a chance of large hail and severe wind gusts with this activity.

... Italy, Western Balkans ...

Fairly low pressure resides in the wake of the Alps, even though synoptic-scale support is too weak for a more pronounced lee cyclogenesis. A plume of regionally steeper lapse rates detaches from the Alps and overspreads Northern italy, Slovenia and Croatia with the postfrontal Northwesterly flow. Insolation and some residual low-level moisture may allow the build-up of a few hundred J/kg of CAPE. Scattered afternoon storms with one or two marginally large hail events are possible, but weak vertical wind shear and limited CAPE should keep the risk too low for a level 1. The late arrival of a small vorticity lobe from the Northwest may keep isolated and non-severe storms going on well into the night.

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